In many instances, such as in automobiles, it is desirable to provide some means to indicate when a remotely located light bulb has burned out. The prior art lamp outage indicators have taken many forms such as a photoelectric type, a fiber optic type, or a current relay type which senses lamp current. All of these types of lamp outage indicators have proved to be unsatisfactory for one reason or another.
One lamp outage indicator which has proved to be satisfactory in many respects is the one disclosed in McNamee, U.S. Pat. No. 3,735,378. Because of supply voltage fluctuations, the system "balances" a right hand array of lamps against a left hand array of lamps and uses a differential amplifier to detect voltage differences across a pair of sensing resistors to provide the lamp outage indication. It is taught in this patent that the sensing resistors may be the resistance of the wiring harness connecting the power source to the lamp array. While this lamp outage indicator operates satisfactorily for its intended purpose, it suffers from the drawback that an even number of arrays must be monitored to provide the voltage independence.
It is also well known in the prior art, as shown by Craford et al, U.S. Pat. No. 3,964,039, to provide a voltage indicator circuit which indicates a voltage below a first level with a first color LED and indicates a voltage above the first level with a different color LED. The switching point between these two LED's is controlled by a switching transistor.